Complete spinal transection is followed by transient depression of reflexes caudal to the lesion ('spinal shock'). Depression is maximal shortly after the transection; thereafter, reflexes gradually increase toward normal and eventually become supranormal. Although post-transectional reflex alterations have been studied in experimental animals and humans using conventional reflex testing techniques, little information is available with respect to the changes at the individual neuronal level. The proposed research aims to identify changes in the properties of spinal motoneurons or in synaptic transmission in motoneurons which may account for the depression and subsequent recovery of reflex activity in the spinal monosynaptic pathway. Intracellular recordings will be made from motoneurons in the cat. Resting membrane potential, firing threshold and input impedance of individual motoneurons will be measured before and after surgical or functional interruption of descending spinal tracts. In addition, by application of the qunatum hypothesis of synaptic transmission to the spinal monosynaptic pathway, the amount of transmitter released from Group Ia afferent terminals will be determined before and after spinal transection. With these criteria, it will be possible to identify whether presynaptic and/or postsynaptic changes underly post-transectional alterations in transmission in the spinal monosynaptic reflex arc.